1. Field of the Invention
The present invention relates to a control device for an internal combustion engine such as a diesel engine, in particular, relating to a control device capable of decreasing a torque difference between lean combustion and rich combustion in an internal combustion engine in order to carry out catalyst control.
2. Description of the Related Art
There is a related-art technique to switch lean combustion and rich combustion of an internal combustion engine in order to supply reducing agent to NOx (Nitrogen Oxide) occuluding and reducing catalyst. The lean combustion provides an excess amount of air rather than that of a stoichiometric air-fuel ratio. The rich combustion provides an excess amount of fuel rather than that of the stoichiometric air-fuel ratio. Such a NOx (Nitrogen Oxide) occluding and reducing catalyst absorbs NOx during the lean combustion, and on the other hand, NOx is reduced and Nitrogen is generated during the rich combustion.
In order to solve world wide environmental issues, the demand to purify exhaust gas emitted from motor vehicles has grown stronger. This demand can also be applied to the NOx purifying technology field and its rapid solution is required.
There are cases where three-way catalyst systems are not used in diesel engines, for example, the NOx (Nitrogen Oxide) occluding and reducing catalyst. This is because using the NOx (Nitrogen Oxide) occuluding and reducing catalyst is one of the most effective techniques, and it is necessary to further develop the technology using the NOx (Nitrogen Oxide) occluding and reducing catalyst.
For example, Japanese patent laid open publication JP 2006-336518 has disclosed a system using an NOx catalyst for judging the state of deterioration of the NOx catalyst based on a value detected by an oxygen concentration sensor, and for setting and adjusting a period of time for the lean combustion.
When compared with a post-injection system (for injecting fuel some approximate time after the fuel combustion has been completed) and an exhaust-gas pipe fuel adding system (for supplying a predetermined amount of fuel into the exhaust-gas pipe), the above systems for obtaining a fuel-rich condition using the rich combustion has the advantages of being able to supply a high-efficiency reducing agent and of using a lower amount of fuel or less fuel. However, the above system has a disadvantage in that it generates a torque difference between the lean combustion and the rich combustion.
FIG. 3A shows the change of an air volume “G” during the lean combustion and rich combustion in a diesel engine as the conventional example. FIG. 3B shows the change of a fuel injection amount “Q” during the lean combustion and the rich combustion in the diesel engine as the conventional example. FIG. 3C shows the change of a torque “T” during the lean combustion and the rich combustion in the diesel engine as the conventional example. FIG. 3D shows the change of air-fuel “A/F” ratio during the lean combustion and the rich combustion in the diesel engine as the conventional example.
The lateral line in FIG. 3A to FIG. 3D indicates the elapse of time “Time”. Each of FIG. 3A to FIG. 3D shows the time zone for the lean combustion, the rich combustion, and the lean combustion, in order, observed from the left side to the right side. The solid line in each figure shows a target value or a theoretical value. The dotted line in each figure designates a detected value or a true value.
In FIG. 3A, the dotted line indicates the detection value detected by an air flow meter, which is shifted from a theoretical air volume. This shifting, namely, the difference between the detected value and the theoretical value is caused by a tolerance or an elapsed deterioration of the air flow meter capable of detecting the air volume.
In FIG. 3B, the dotted line shows the actual fuel injection amount which is shifted from the target injection amount by the tolerance or the elapsed deterioration of the air flow meter.
It will now be explained what the solid line indicates in each of FIG. 3A to FIG. 3D without considering the value indicated by the vertical line.
FIG. 3D shows the change of the A/F ratio value. The A/F ratio value exceeds the value “14.5” during the lean combustion, and on the other hand, becomes lower than the value “14.5” during the rich combustion.
FIG. 3C shows the change of the torque value during the lean combustion and the rich combustion. Because the switching of the lean combustion and the rich combustion does not affect the torque value (such a torque value is requested though the accelerator pedal of the vehicle by the driver), the target value or the torque to be requested which is designated by the solid line has a constant value, which is not changeable.
As described above, it is necessary to change both the A/F value and to keep the constant torque value in the switching process between the lean combustion and the rich combustion.
The air volume G and the injection amount Q designated by the solid lines in FIG. 3A and FIG. 3B are the target air volume and the target injection amount, respectively, which are calculated by an electrical control unit (ECU) in order to have the A/F value in each of the lean combustion and the rich combustion, and to keep the constant torque value shown in FIG. 3C. The ECU controls an intake throttle valve and an EGR valve so that the target air volume becomes equal to the true air volume detected by the air volume detection means. In addition, the ECU calculates the injection period of time according to the fuel pressure and instructs the injectors of the internal combustion engine so that the current injection amount becomes equal to the target injection amount. In theory, it should be able to achieve the constant torque shown in FIG. 3C and the changeable A/F value shown in FIG. 3D by the air volume “G” shown in FIG. 3A and the injection amount “Q” shown in FIG. 3B.
However, it is difficult in actual cases to precisely perform the above theoretical operation. The dotted line shown in FIG. 3A indicates the detected air volume which is shifted from the theoretical or target air volume designated by the solid line.
The dotted line shown in FIG. 3B indicates the actual fuel injection amount which is shifted from the target injection amount designated by the solid line.
The output torque generated by the internal combustion engine is determined by the fuel injection amount during the lean combustion, and is also determined by the air volume during the rich combustion. When the injectors inject the practical amount of fuel as designated by the dotted line shown in FIG. 3A, and the air flow meter detects the practical air volume as designated by the dotted line shown in FIG. 3B, the torque difference between the lean combustion and the rich combustion is generated because the actual torque designated by the dotted line shown in FIG. 3C fluctuates, namely, it becomes difficult to keep the constant torque value through the lean combustion and the rich combustion. So, there is a possibility of decreasing the drivability of the vehicle when this torque difference becomes large, and it is desirable to decrease it as low as possible. No related-art techniques using NOx (Nitrogen Oxide) occluding and reducing catalyst as well as JP 2006-336518 consider such a problem regarding the torque difference between the lean combustion and the rich combustion.